14 MICRO-ORGANISMS AND FERMENTATION. 



considerably higher temperature ; the latter alternative is 

 specially necessary when dry heat is used. 



(a) Sterilisation of Glass and Metal Objects. Sterilisation, 

 properly so called, must always be preceded by a thorough 

 mechanical, and often by a chemical, cleansing. Articles of 

 daily use in the laboratory, such as spatulas, needles, wires, 

 etc., are heated directly in a flame, and allowed to cool in a 

 space free from germs. Heavy pieces of apparatus, however, 

 do not admit of this treatment ; harm may be done by over- 

 heating while ensuring that every part of the object has been 

 sufficiently heated, or the number of objects to be sterilised 

 may be so great that it would take too much time to treat 

 each singly. The apparatus must in this case be placed in 

 special sterilisation ovens, where it is exposed for some time 

 to a temperature at which it is believed that all germs will be 

 destroyed. 



Dry or moist heat may be used according to the. nature 

 of the article. Dry heat is a much weaker disinfectant than 

 moist heat at the same temperature. To make certain that 

 all germs are killed when using a hot air steriliser, the air 

 must be raised to a temperature of 150 to 160 C., and the 

 articles must be subjected to this heat for one to two hours. 

 Some objects are wrapped in paper, others (e.g., flasks) are 

 closed with a cotton wool plug, which should be covered over 

 with filter paper. If moist heat is required, the object can 

 either be boiled in a water bath, or, better still, subjected 

 to the action of steam. It is obviously of importance to see 

 that the air is completely driven out, so that it cannot form a 

 protecting layer, and prevent the steam from coming into 

 contact with the object. 



Either a current of steam may be used, or steam under 

 pressure. In the first case, the apparatus is placed in a vessel 

 provided with a perforated false bottom, with a sufficient 

 quantity of vigorously boiling water below it. The steam 

 escapes slowly, as the lid of the vessel is not air-tight, and the 

 apparatus is gradually raised to the boiling point. By boiling 

 in steam at 100 C. all vegetative forms are probably killed, 

 together with many spores of bacteria and other resistant 

 forms if the treatment is continued for an hour. 



